The presence and concentration of analytes, as well as the progress and efficiency of chemical reactions, are typically measured directly through optical monitoring if a reaction produces an observable change in light-absorption characteristics, or measured indirectly by observing, for example, changes in mass or volume. Many of the methods typically employed require attachment of a label compound whose properties-fluorescent, radioactive, chemiluminescent, or absorbing, for example-enable sensitive detection. These methods, however, typically require development of label reagents, add steps to the detection process, and modify the analyte. In the absence of label compounds, conventional measurements operate on a gross scale, and as a result require substantial amounts of analyte.
Enhanced sensitivity has recently been achieved using small micromachined cantilevers and flexural plate wave (FPW) sensors that facilitate the monitoring of chemical reactions and interactions on a microscopic scale. In the cantilever sensor, a selective coating applied to a face of the cantilever transduces the chemical reaction into a mechanical stress. This stress may be detected with a high degree of sensitivity. Cantilever arrangements may, however, be difficult to manufacture and to operate due to the small size and fragility of the fingers and due to the need to separate analytes from the readout mechanism. Because the cantilevers are delicate, applying the selective coatings may also be difficult. To separate the analytes from readout electronics, optical readouts employing reflection may be used. Cantilever-based approaches have achieved success primarily in specialized laboratories with personnel trained to handle the nuances of such devices.
FPW systems may utilize a diaphragm that is acoustically excited by interdigitated fingers to establish a standing wave pattern. The diaphragm is coated with the selective material, and interaction of analytes with the coating increases the effective thickness of the diaphragm, thereby affecting the frequency of the standing wave so as to indicate the degree of interaction. Because these devices are constructed of conducting, mechanical, and piezoelectric layers, bimetallic effects may produce unwanted temperature sensitivity. To reduce thermal distortions, FPW sensors are typically run at high resonant frequencies. Unfortunately, the high operating frequency itself limits sensitivity, in addition to requiring somewhat complex electronics.
Another approach transduces a chemical reaction into a mechanical stress applied to a diaphragm. The diaphragm is suspended in a narrow substrate cavity, and a selective coating that reacts to an analyte is applied to the diaphragm within the cavity. Because of the placement of the coating within the cavity, however, applying the coating consistently and evenly to the entire surface of the diaphragm is difficult, as is characterizing or modeling the unevenly applied coating. Further, the analyte of interest may take additional time to diffuse into the cavity from the outside environment, increasing the time required for measurements.